Based on the biblical story of Adam and Eve's first two sons, Cain and Abel is about Cain's jealousy towards his brother Abel. Lee Cho-in is a very gifted doctor who has everything that he wants whereas his older brother, Seon-woo, is jealous of all the attention that Cho-in receives. Seon-woo blames his brother for taking everything good in his life away from him. Seon-woo blames Cho-in for getting their father's love, getting more recognition as a doctor, and for stealing the woman he loves.